Sailing to Bermuda is a bit different than sailing the Caribbean. With no storms around, the Caribbean is like sailing in a big warm water bathtub.
Bermuda on the otherhand is several hundred miles out into the open Atlantic. Whether you sail from NY or Boston, you do sail the open waters of the Atlantic for 2 days in each direction. Second factor with sailing to Bermuda is the ships tend to be amoung the smallest out there these days (ie Zenith, Horizon, Majesty etc.). Combine the two things and you will get what some people feel is a bumpy ride.
On my last cruise while some folks complained about rockin' and rollin' others thought it was the smoothest ride they ever had. It's all relative. Based on storm frequency, you will more likely get a better ride in May and June than Sept and Oct.
Waiting on the other end is perhaps the single best cruise destination in the world, Bermuda.